The invention relates to a method and circuit for controlling a rectifier, which is connected to an AC power supply system on its input side and a pulse-controlled inverter on its load side.
In drive technology, there is frequently a requirement for an uncontrolled, power supply system-commutated rectifier of simple design in order to feed a DC intermediate circuit in a drive converter from a three-phase power supply system, in which case the requirement to draw and/or to supply a current which is as sinusoidal as possible from and to the power supply system is not of importance. Since the converter is not only intended to drive the three-phase motor which is supplied with a variable three-phase voltage and at a variable frequency, but also to brake it a current flow must, however, be possible in both directions.
In the braking mode, energy flows from the motor via the load-side inverter into the DC intermediate circuit, and from there into the power supply system. This is not possible in the case of uncontrolled rectifiers which contain only diodes. Electronically switchable braking resistors, which convert the braking energy that is produced to heat, must therefore be arranged in the power supply input for drive converters with diode bridges.
However, if the aim is to feed the braking energy back into the supplying power supply system, it is possible to provide a self-commutated pulse-controlled converter with semiconductor switches which can be switched off, in particular insulated-gate bipolar transistors (IGBTs) instead of a rectifier. This self-commutated pulse-controlled converter can be operated as a rectifier or as an inverter. This self-commutated pulse-controlled converter is therefore also referred to as a self-commutated, pulsed, input/output feed unit, or as an active front end (AFE). The power supply system current is virtually sinusoidal as a result of the use of an active front end, that is to say the reactions on the power supply system are minimal.
In comparison to an unregulated input-feed unit, the intermediate-circuit voltage can be regulated by means of the self-commutated, pulsed input/output feed unit. The disadvantage is the high costs and the high level of complexity. In the event of stringent requirements relating to the reactions on the power supply system or when a high braking power is produced, there is no other alternative to the use of an input/output feed unit such as this, even if there is no need for a regulated intermediate-circuit voltage.
DE 35 39 027 A1 discloses a further embodiment of a drive converter, which has an uncontrolled rectifier on the power supply system side and two diodes (which are electrically connected in series) with electronically controllable switches connected in parallel with them, for each voltage phase. Furthermore, this drive converter has an auxiliary rectifier, which is terminated on the DC-voltage side by a high-impedance burden resistor and, in series with each of its auxiliary diodes, has a current sensor, in particular an optocoupler with a downstream amplifier, which is used to drive an electronically controllable switch. This auxiliary rectifier generates control signals in such a manner that each electronically controllable switch is switched on in synchronism with the on-phases of the associated power supply system-commutated diodes. The rectifier in the drive converter is thus forward-biased bidirectionally for both current directions, and is nevertheless of the power supply system-commutated, uncontrolled type. The advantage of this drive converter is that the rectifier does not require any complex control and regulation electronics. Although the rectifier uses valves which can be switched off, the DC voltage is as high as that in the case of an uncontrolled rectifier. The valves which can be switched off switch only at the power supply system frequency, and after being switched off the voltage across them rises only slowly, so that thyristors which can be switched off are also suitable for use as electronically controllable switches.
The publication “Fundamental Frequency Front End Converter (F3E)—a DC-link drive converter without electrolytic capacitor” by Kurt Göpfrich, Dr. Carsten Rebbereh and Dr. Lothar Sack, PCIM 2003, Nürnberg, May 2003 discloses a drive converter which has an uncontrolled rectifier on the power supply system side, and a self-commutated pulse-controlled converter on the load side. The two converters are connected directly electrically in parallel on the DC voltage side, that is to say neither an intermediate-circuit capacitor nor an intermediate-circuit inductor is arranged in the so-called DC voltage intermediate circuit. Each diode in the rectifier on the power supply system side is connected electrically in parallel with an electronically controllable switch, in particular an insulated gate bipolar transistor (IGBT). A filter circuit, comprising filter inductors and filter capacitors, is arranged between the power supply system connections and the input connections of the converter on the power supply system side. A comparator circuit is provided for generation of control signals for these electronically controllable switches, and is linked on the input side to the power supply system which feeds it. This comparator circuit and its operation are dealt with in detail in DE 199 13 634 A1, so that it will not be explained in any more detail at this point.
The switching operations of the electronically controllable switches for the uncontrolled rectifier on the power supply system side when the power supply system-commutated converter for the drive converter is being controlled on the basis of DE 35 39 027 A1 or the PCIM publication are carried out solely on the basis of the general criterion of the power supply system voltages in the power supply system which feeds them. The direct current flowing between the converter on the power supply system side and the converter on the load side is in this case ignored. This current is impressed on the load side, and may be positive, negative or zero depending on the operating state and time. Furthermore, there is no insurance that, during the commutation of the current from one phase to a subsequent phase, that the switch carrying the current and the switch to which the current is being transferred will switch at the same time. This is caused by the determination of the zero crossing of the associated line-to-line voltage which, for example, is in each case present across a filter capacitor in the input-side filter circuit, and the determination of the time of natural commutation. A plurality of states therefore exist as a function of the type of operation, during the commutation process. For example, a delay in switching on a switch to which the current is being transferred during the commutation process in the case of a negative direct current causes disturbance excitation of the entire system in the drive converter in the event of a line-to-line voltage which is greater than, equal to or less than zero, and leads to voltage spikes across the filter capacitors and the electronically controllable switches. These overvoltages that occur must be taken into account in the design of the drive converter.